PEDESTRAIN HEAD IMPACT SIMULATION USING LS-DYNA

PEDESTRAIN HEAD IMPACT SIMULATION USING LS-DYNA

 

AIM: To model spot weld as beam and hexa elements and create a complete simulation file for crash analysis from the given FE model of assembly of parts and produce the following deliverables.

 

1. To perform the Head Impact Simulation and calculate the Head Impact Criterion (HIC) value for the following cases.
2. Simple head model impacting against rigid wall
3. Child head form dummy model impacting against rigid wall
4. Child head form dummy model impacting against hood

INTRODUCTION:

 

Pedestrian protection CAE is a part of vehicle safety engineering from a crash perspective. In pedestrian protection, we look for imparting minimum injury to head, upper leg (thigh), and lower leg (including knee) in case of an accident with a human subject. These tests are usually done at a lower velocity from 20-40 kmph as accidents tend to result in more severe circumstances above these velocities and braking in case of an accident often results in collision at low velocities. The standard pedestrian test procedure by Euro NCAP is as shown in the fig.1.

In this project, a children's head form model is provided. An impact simulation is created to replicate a scenario where this head form will impact on a car bonnet. The bonnet is considered as a rigid wall for the first and second case, while for third case a meshed hood model is used with elasto-plastic material card. From the simulation, the head impact coefficient is calculated manually and with the help of LS-PrePost.

 

HIC, Head Injury Criterion/Coefficient is a quantification of head injury. A small HIC value doesn’t mean that the head injuries will be of low levels. Likewise, a high HIC doesn’t imply high level injuries. The real meaning is that with higher HIC values, the higher the probability of getting high level head injuries. The HIC is the maximum value over the critical time period t to t for

the expression.

• HIC=max (t_1, t_2) {[1/ (t_2-t_1) ∫_(t_1) ^(t_2) a(t) dt] ^2.5 (t_2-t_1)} `
• HIC(d)=0.75446 (text (Free Motion Head form HIC)) +166.4`

 

At HIC=650,

90% probability of level one,

55% of level 2,

20% of level 3,

5% of level 4.

 

AIS-Abbreviated Injury Scale

 

1. Slight damage to brain with headache, dizziness, no loss of consciousness, confusions.
2. Concussion with or without skull fracture, less than 15 minutes of unconsciousness, detached retina, face and nose fracture.
3. Concussion with or without skull fracture more than 15 minutes of unconsciousness without severe neurological damage, multiple skull fracture, loss of vision, multiple facial fracture, cervical fracture without damage to spine.
4. Multiple skull fracture with severe neurological damage.

 

Note: The unit system used is kg-mm-ms.

 

PROCEDURE:

 

Case (1): Simple head model impacting against rigid wall:

The given LS-Dyna keyword file of simple head FE model is opened in LS-PrePost using option File>Open>LS-Dyna Keyword File.

Rename the Title as required.

1. PART DEFINITION

 

Section properties:

The section properties of simple head are assigned as shell element with 2.5 mm thickness and ELFORM=2, Belytschko-Tsay

element formulation.

 

Material properties:

MAT24 (Piece wise linear plasticity) material card is used to assign the steel material properties to the simple head model. The

MAT24 represent Piecewise linear isotropic plasticity. With this material model it is possible to consider the effect of the strain

rate.

Assign the Properties to the Part

2.BOUNDARY CONDITIONS:

 

Initial Velocity

The simple head model is assumed to be impacting the rigid wall at a velocity of 40 kmph i.e., 11.11 mm/ms in negative Z direction

towards rigid wall.

Rigid wall planar

The rigid wall is created at a distance of 15 mm from the bottom of simple head model.

Contact details:

The contact type selected is AUTOMATIC_SINGLE_SURFACE. It is quite helpful to apply this contact method in the crash models because all the elements are included in one single set and LS-DYNA considers also when a part comes into contact with itself. The FS and FD that are static and dynamic friction coefficient with a value of 0.02 is entered in the contact card. The

simple head form model is the slave (SSID=5).

3. CONTROL AND DATABASE OPTIONS:

 

CONTROL FUNCTION

 

Control termination

The control termination function is enabled to specify the end time of simulation. The termination time is set for 20 ms.

 

DATABASE OPTIONS:

Database d3plot

The time step value of 0.5 ms is given for the BINARY_D3PLOT and DATABASE_ASCII option for GLSAT, NODOUT and RWFORC

DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.

DATABASE_HISTORY_NODE card is used to compute the HIC value of a 68991 node in the model.

The keyword file created is checked for errors using the option keyword manager>model check. The keyword file is saved using

‘. dyn’ or ‘.k’ extension and is made to run in the solver by getting normal termination message.

 

Case (2): Child head form dummy model impacting against rigid wall:

 The given LS-Dyna keyword file of simple head FE model is opened in LS-PrePost using option File>Open>LS-Dyna Keyword File. Rename the Title as required.

 UNIT SYSTEM USED IN PEDESTRIAN FILE IS

MASS

LENGTH

TIME

FORCE

STRESS

ENERGY

DENSITY

YOUNG's

35MPH     56.33KMPH

GRAVITY

kg

mm

ms

kN

GPa

kN-mm

7.83e-06

2.07e+02

15.65

9.806e-03

 

 .The standard dummy model of child head form was provided with necessary keyword for impact simulation as shown in the fig.11.

For case (2) impact simulation, the provide standard dummy model of child head form has to be rotated about 50⁰ along Y-axis

to replicate the Euro NCAP impact simulation as shown in fig.1, by pulling it into a new keyword file created by LS-PrePost by

using *DEFINE_TRASFORMATION and *INCLUDE_TRANSFORM as shown in the fig. and fig.

Using DEFINE_TRANSFORMATION card with TRANID=1, the dummy head form model is rotated by OPTION>ROTATE along

Y-axis (A2=1) by 50 (A7=50).

INCLUDE_TRANSFORM card is used to pull the rotated standard dummy head form model file to the main file. The main file is

Saved as required name with ‘.dyn’ extension. So, we can differentiate within the two files.

 

Note: Filenames and pathnames are limited to 236 characters spread over up to three 80-character lines.

 

2. BOUNDARY CONDITION

Initial velocity:

As per Euro NCAP standards, the initial velocity is taken as 40 kmph i.e, 11.11 mm/ms at an angle 50. The vertical and horizontal

components of the velocities are -7.14 mm/ms and -8.51 mm/ms respectively.

Rigid wall planar

The rigid wall is created at a distance of 15 mm from the bottom of simple head model.

Contact details:

The contact type selected is AUTOMATIC_SINGLE_SURFACE. It is quite helpful to apply this contact method in the crash models because all the elements are included in one single set and LS-DYNA considers also when a part comes into contact with itself. The FS and FD that are static and dynamic friction coefficient with a value of 0.02 is entered in the contact card. The

simple head form model is the slave (SSID=5).

3. CONTROL AND DATABASE OPTIONS:

 

CONTROL FUNCTION

 

Control termination

The control termination function is enabled to specify the end time of simulation. The termination time is set for 20 ms.

DATABASE OPTIONS:

Database d3plot

The time step value of 0.5 ms is given for the BINARY_D3PLOT and DATABASE_ASCII option for GLSAT, NODOUT and RWFORC

DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.

DATABASE_HISTORY_NODE card is used to compute the HIC value of a 68991 node in the model.

The keyword file created is checked for errors using the option keyword manager>model check. The keyword file is saved using

‘. dyn’ or ‘.k’ extension and is made to run in the solver by getting normal termination message.

 

 

Case (3): Pedestrian Headform model impacting against Engine Hood:

 

Review the unit system of file which we are going to include in the Engine hood file. So, open the pedestrian file

 

MASS

LENGTH

TIME

FORCE

STRESS

ENERGY

DENSITY

YOUNG's

35MPH     56.33KMPH

GRAVITY

kg

mm

ms

kN

GPa

kN-mm

7.83e-06

2.07e+02

15.65

9.806e-03

 

Import the main file in which you want to include all the other files. Here it is the file on meshed hood is imported.  

 

As this file not consist of all the cards like Section card, Material Card. We have to create that card and save or overwrite the file as it is. Now you can open the file in LS Prepost.

 

Now as we know the pedestrian headform provide is in a standard orientation. We cannot modify the standard as it for our sake. It’s a standard Practice that we have to transform the required file according to our use and include the same file as a reference in our main file which we are going to write.

 

The standard dummy model of child head form was provided with necessary keyword for impact simulation as shown in the fig.11.

For case (2) impact simulation, the provide standard dummy model of child head form has to be rotated about 50⁰ along Y-axis

to replicate the Euro NCAP impact simulation as shown in fig.1, by pulling it into a new keyword file created by LS-PrePost by

using *DEFINE_TRASFORMATION and *INCLUDE_TRANSFORM as shown in the fig. and fig.

Now we have to save the file and close the LS prepost and again open the same file. You will see the pedestrian headform is included to the main file with the given transformation as per our requirement.

2. BOUNDARY CONDITION

Initial velocity:

As per Euro NCAP standards, the initial velocity is taken as 40 kmph i.e, 11.11 mm/ms at an angle 50. The vertical and horizontal

components of the velocities are -7.14 mm/ms and -8.51 mm/ms respectively.

 

Contact details:

The contact type selected is AUTOMATIC_SINGLE_SURFACE. It is quite helpful to apply this contact method in the crash models because all the elements are included in one single set and LS-DYNA considers also when a part comes into contact with itself. The FS and FD that are static and dynamic friction coefficient with a value of 0.02 is entered in the contact card. The

simple head form model is the slave (SSID=5).

3. CONTROL AND DATABASE OPTIONS:

 

CONTROL FUNCTION

 

Control termination

The control termination function is enabled to specify the end time of simulation. The termination time is set for 20 ms.

DATABASE OPTIONS:

Database d3plot

 

The time step value of 0.5 ms is given for the BINARY_D3PLOT and DATABASE_ASCII option for GLSAT, NODOUT and RWFORC

DATABASE_EXTENT_BINARY card with STRFLG =1, is used to compute the elastic strain in the model.

DATABASE_HISTORY_NODE card is used to compute the HIC value of a 3840 node in the model.

RESULTS:

The D3plot output file is opened in LS-PrePost using option File>open>LS-Dyna binary plot.

1. The animation of Von-Mises stress and Effective plastic strain contour is as shown below.

Von-Mises stress

Effective Plastic Strain

2. Stress plots

v-m stress plot for an element:

 Von-Mises stress

Effective Plastic Strain

2.Stress plots

v-m stress plot for an element:

 Von-Mises stress

Effective Plastic Strain

 

 

2.Stress plots

v-m stress plot for an element:

 

1. In the first case, during the instant of impact the maximum v-m stress developed is 76 MPa in the element 15155 which is

beyond the yield stress value of 355 MPa.

2. In the second case, during the instant of impact the maximum v-m stress developed is 95 MPa in the element 3365 and

reaches a peak value of 12.5 MPa at 3 ms and later the value of v-m stress reduces due to the bouncing back of the headform

from rigidwall.

3. In the third case, during the instant of impact the maximum v-m stress developed is 217 MPa in the element 2540235 and later it. fluctuates steadily until the headform bounces back from the hood. The v-m stress induced in the element for third case is less compared to second case because the hood gets deformed due the impact and absorbs the energy unlike the rigid wall which does not get deformed during impact.

 

3. Head Injury Criteria (HIC15)

 

The resultant acceleration plot with HIC value for case (1)

The resultant acceleration plot with HIC value for case (2)

The resultant acceleration plot with HIC value for case (3)

From the plot, the HIC value for first and second case is beyond the safe value because the head model is impacting non deformable rigid wall. The HIC value obtained from LS-PrePost for third case is 236.5 and HIC value is 344.8 which is less than the safe HIC value of 650 as per Euro NCAP Protocol because the hood absorbs the impact energy and gets deformed.

 

 

Manual calculation of HIC value:

 

The expression to calculate HIC value is,

From the plot,

The average value of acceleration for the time interval of t =2.5 ms and t =17.5 ms is 48mm/ms² from the graph

The HIC₁₅ value obtained from LS-PrePost for third case is 236.5 and HIC_d value is 344.8.

The HIC₁₅ value obtained from manual calculation for third case is 239.438 and HIC_d value is 347.046. There is a small variation in the HIC value obtained from LS-PrePost and manual calculation which is acceptable.

 

CONCLUSION:

 

1. The impact simulation of pedestrian headform is created for a simple case scenario to a complex real time scenario i.e.,

case (1): Simple head model impacting against rigid wall

case (2): Child headform model impacting against rigid wall

case (3): Child headform model impacting against hood.

2. The computational time required for simple case is less compared to real time case scenario but the accuracy of the results is

poor for simple case.

3. There is a variation in the values for stress/strain and HIC for all the cases because of the difference in material properties

and boundary conditions.

4. The HIC value obtained for case 3 was within the standard safe HIC value of EURO NCAP protocol.

 

 

UPLOADS

CASE1 - only keyword file name - Simple_Head_Impacting_On_Rigid_Wall

CASE2 - Two files on keyword and One PEDESTRIAN_HEADFORM - main.dyn

CASE3 - Two files on keyword and One PEDESTRIAN_HEADFORM - Meshed_Hood

 

GOOGLE DRIVE LINK

https://drive.google.com/file/d/1kgoElVprwtwkhbU9yqlK69qkf7An--xy/view?usp=sharing